Kelman, I. 1996. Modifying and Testing a Length Modulated Radiometer Used for Measuring Atmospheric Carbon Monoxide and Methane. Bachelor of Applied Science dissertation, University of Toronto, Canada, full text (6,000 kb in PDF).

Abstract

In order to understand the impact and mitigation of anthropogenic influences on the atmosphere's properties, continual and global observations are an immense asset. The tropospheric concentrations of carbon monoxide (CO) and methane (CH4) are particularly important for environmental modelling, since they play key roles in atmospheric chemistry. Correlation radiometry is often used for such observations, and a new form, called length modulation radiometry, has been developed at the University of Toronto.

A ground-based length modulated radiometer (LMR) for observing CO and CH4 was built and tested with the ultimate goal of measuring CO at the 4.7 micrometer and 2.4 micrometer bands to a precision of +/-10% and CH4 at the 2.3 micrometer band to a precision of +/-1%. The work presented in this report involved modifying and testing this LMR. Initially, the instrument was tested to ensure that results from previous work were repeatable. Optimal setup parameters to attain the minimum imbalance were the same as those obtained before, and the LMR's response to various nitrogen and CO pressures matched the theoretical predictions.

Several modifications were made to the LMR to improve its performance. A fibre optic cable to direct the radiation source into the LMR was added, a rotating vane chopper was used, and the optical setup was improved to minimize the imbalance. As well, the digital signal processing software and hardware were upgraded to allow more efficient, and more powerful, data processing and analysis. Following these changes, the optimal parameters for operating the LMR were re-established. The changes were also implemented to permit the addition of a second detection channel to the LMR. This second channel will allow the reduction of noise in the data stream from solar radiation. The work presented in this report seems to demonstrate that the LMR is likely to be an effective ground-based instrument for measuring CO and CH4.

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